The present invention relates to an ion microbeam apparatus which is used for the lithography or implantation utilizing an ion microbeam, and particularly to an ion microbeam apparatus which makes it possible to adjust the beam diameter as well as to effect auto-focusing.
FIG. 1 shows the fundamental construction of a conventional ion microbeam apparatus. Such a conventional apparatus has been disclosed, for example, in a literature "Shinku (Vacuum)", Vol. 25, No. 6, pp. 439-447. A voltage of about 10 KV is applied across an emitter 1 of an ion source and an ion-extracting electrode 2 by an ion-extracting power supply 3 in order to extract ions from the emitter 1. The ions extracted from the emitter 1 are accelerated to about 100 KV by an ion-accelerating power supply 4. The ions are focused on a sample 7 by electrostatic lenses 5a, 5b, 5c. The focused beam performs the scanning owing to a deflector 8 controlled by a scan controller 10. The electrostatic lens 5 consists of two or more electrodes. In the following description, use is made of an Einzel lens which consists of three electrodes. In this electrostatic lens, the electrodes 5a, 5c at both ends are maintained at ground potential, and a high voltage of about 80 KV is applied to the central electrode 5b from a lens power supply 9.
The sample 7 irradiated with the ion beam emits secondary electrons 11 that will be detected by a secondary electron detector 12. A secondary-electron-image observation unit 13 under control of the scan controller 10 observes the data related to the surfaces of sample such as ruggedness of surfaces, and monitors the diameter of the beam. In the ion microbeam apparatus as mentioned above, the beam having a fine diameter is obtained in observing the samples. Namely, the beam is correctly focused on the sample 7 by the lens 5. For this purpose, the lens voltage V.sub.L is scanned over a range of about 100 volts to hover around an optimum lens voltage, and a minimum beam diameter is looked for while monitoring the beam diameter.
According to the conventional apparatus, however, a time of longer than about 10 seconds is required to scan a range of as narrow as about .+-.100 volts maintaining a precision of 0.5 volt or smaller with the lens power supply 9 of a voltage of as high as about 100 KV, because of its large time constant. Therefore, it is difficult to accomplish the focusing at a reduced period of time. In the conventional apparatus, this difficulty is always involved even when the lens system of a plurality of stages is employed, or even when a mass separator is installed in an ion optical system to separate ions into those of different kinds.